Lecture 4

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Chapter 2

Soil Morphology


Morphology refers to form, shape, appearance or structure. As we study soil morphology we find ourselves at a loss for words--so we make some words up. This lecture is a parallel study of the form and structure of soils and the words and symbols used to describe those forms and structures.

  1. Uppercase letters, O, A, E, B, C, and R are used to designate master horizons. The symbol "O" represents an organic horizon. Most soils have no O horizon, they are however, common in forests and swamps and other places where organic production exceeds decomposition. The symbol "A" means topsoil. Most soils have an A horizon, but it could have eroded off exposing a subsoil horizon. Some very young soils have not yet had time to form an A horizon. The symbol "B" means region of accumulation. Salts and clays accumulate below the A horizon in a B horizon. Most soils have a B horizon. The symbol "E" means eluvial or washed out. A small fraction of soils have an E horizon, usually between the A and B horizons. The symbol "C" means parent material, i.e., unconsolidated mineral material that has not been affected appreciably by weathering, leaching, etc. The symbol "R" means bedrock, i.e., mineral material that cannot be considered unconsolidated. It is too hard for roots to penetrate.

  2. Various special cases exist. If two or more subhorizons exist within a master horizon, they can be designated by arabic numerals, such as B1 and B2, indicating a difference between two horizons within a B horizon. If a discontinuity occurs so that a master horizon is comprised of two or more distinct parent materials, this can be designated by preceding letters with Arabic numerals, such as 1B and 2B. Normally "1" before the first B is understood, and is omitted resulting a B horizon, followed by a 2B horizon. These special cases are not limited to B horizons.

    Transition zones might have properties of two master horizons, and can be called AB, BA, BC, etc. Boundaries between horizons are often described. Boundaries separating horizons may be smooth, tongued, wavy, indistinct, or have other properties worthy of note.

  3. Lower-case letters are used to describe the master horizons. Think of these descriptions as adjectives. For example, a plowed A horizon may be designated an "Ap" horizon, with "p" indicating that the soil has been plowed and is therefore not in a natural condition. As another example, a B horizon dominated by deposited clay is a "Bt" horizon. The "t" stands for the German word for clay. Many of these lower-case adjectives are used, a few of which are explained in the book (see Table 2-2 in the textbook). ***A typical soil in a semi-arid region may have a vertical sequence of horizons such as: Ap/Bt/Ck. In a cool forest the sequence may be: Oe/E/Bs/Cg. Many possible combinations of horizon sequences exist.

  4. Diagnostic epipedons are surface indicators of soil properties. Epipedon means topsoil, or surface horizon. Some epipedons are common and important to soil students. These are: mollic (a thick, dark, fertile topsoil, common in grasslands), ochric (a thin or light-colored topsoil, common in arid lands), umbric (an acidic, dark topsoil, common in the subtropics), and histic (an organic topsoil, not common but important for wetlands and other special cases).

  5. Some specific kinds of subsoils are common and important. These are: argillic (a clay accumulation, very common in temperate zones), cambic (a weakly developed B horizon, often found in young soils), natric (a problematic clay accumulation with high sodium contents, common in the presence of a high water table), spodic (an acidic accumulation of humus or oxides, common in cool forests growing in sandy soil, usually found below an E horizon), calcic (an accumulation of CaCO3, common in arid & semi-arid lands).

  6. Soils can be degraded or destroyed. Degradation occurs through nutrient depletion as we break the nutrient cycle by harvesting crops and removing them from the field. Erosion of soil can be catastrophic as the topsoil blows or washes away, leaving only lower-quality B horizon as a plant growth medium. Humus levels can diminish due to decomposition if the soil is used to produce annual crops and is left bare much of the year. Many pollutants, including salt, can also degrade or destroy a soil.

  7. Soil quality can be assessed. We frequently test soil to determine its suitability for agricultural, environmental or engineering purposes. Often these tests are on-going as part of a monitoring program. One can test: physical properties (texture, infiltration, aeration, temperature) chemical properties (organic matter, nutrients, pH, salinity, clay type), and biological properties (microbial biomass, mineralizable substrate).

  8. Soils are mapped as unique individuals. The smallest individual that can be considered a unique soil is called a pedon. A pedon must be at least 1 m2 in area. The USDA uses explicit guidelines to differentiate soil units. They developed a system of Soil Taxonomy in the early 1970s. This system continues to evolve. The individual mapping units of soils are called soil series and phases of series. A series has a name like the "Victoria clay". A phase is a subdivision of a series, used only where necessary in soils mapping.


Students are encouraged to look up the following vocabulary words in the textbook glossary and to browse the following web site.


soil horizon


Web site

The NSCSS Pedology Page has lots of useful information and links to other sites. URL: www.nscss.org/ped.html


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